Dennis Crowley, the founder of the social media site Foursquare, wrote a public apology on WCVB’s website after causing an outcry when his wife, Chelsa, was spotted running the Boston Marathon with another runner’s bib number.

Kathy Brown of Framingham, a registered marathon runner, logged on to a website to see pictures of her running on Monday.

Brown told WCVB, “I opened it up looking for the pictures of me and there were some in there and they came out really good, but then I saw this other woman. It was pretty easy to figure out that someone just made a fake bib.’’

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Crowley, a Medway native, wrote, “Using a duplicate number to get Chelsa into the starting corral with me was wrong.’’

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